Chapter 7
Highway Pavement Materials and Design
7.1 Introduction,
A highway pavement is composed of a system of overlaid strata of chosen
processed materials that is positioned on the in-situ soil, termed the subgrade.
Its basic requirement is the provision of a uniform skid-resistant running surface
with adequate life and requiring minimum maintenance. The chief structural
purpose of the pavement is the support of vehicle wheel loads applied to the
carriageway and the distribution of them to the subgrade immediately under-
neath. If the road is in cut, the subgrade will consist of the in-situ soil. If it is
constructed on fill, the top layers of the embankment structure are collectively
termed the subgrade.
The pavement designer must develop the most economical combination of
layers that will guarantee adequate dispersion of the incident wheel stresses so
that each layer in the pavement does not become overstressed during the design
life of the highway.
The major variables in the design of a highway pavement are:
The thickness of each layer in the pavement
The material contained within each layer of the pavement
The type of vehicles in the traffic stream
The volume of traffic predicted to use the highway over its design life
The strength of the underlying subgrade soil.
There are three basic components of the highway pavement, general definitions
of which are given here. (More detailed descriptions of their composition appear
in the explanations of the two major pavement types later in the chapter.)Foundation
The foundation consists of the native subgrade soil and the layer of graded stone
(subbase and possibly capping) immediately overlaying it. The function of the
subbase and capping is to provide a platform on which to place the roadbase
material as well as to insulate the subgrade below it against the effects of
inclement weather. These layers may form the temporary road surface used
during the construction phase of the highway.